RF Cafe Software
About RF Cafe
1996 - 2022
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...
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All RF Cafe quizzes would make perfect fodder for employment interviews for technicians or engineers - particularly those who are fresh out of school or are relatively new to the work world. Come to think of it, they would make equally excellent study material for the same persons who are going to be interviewed for a job.
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Note: Many answers contain passages quoted in whole or in part from the text.
This quiz is based on the information presented in Stimson's Introduction to Airborne Radar, Third Edition, by Hugh D. Griffiths, Christopher Baker, and David Adamy.
The publisher was not kidding when he stated, "recognizing that people who have an interest in airborne radars love airplanes, dispersed through the book are photos and renderings of radar-bearing aircraft from all over the world." If for no other reason, Stimson's book is a good one to leave on your desk or coffee table as a way-cool picture book for your friends to peruse while visiting. Introduction to Airborne Radar is in every sense a text designed to turn the electronics neophyte who has a reasonable grasp of algebra and physics into a technician who possesses a working knowledge of airborne radar (and radar in general) transmitter, receiver, antenna, and processor systems. The authors teach from first principles about antenna radiation, the decibel, frequency conversion, filtering, signal processing, target acquisition and tracking, spread spectrum, Doppler shift, radar cross section, electronic countermeasures, clutter cancelling, platform configurations, and much more. This book is not like the equally vaunted Skolnik Radar Handbook that is chock full of equations and theoretical discussions, known to make otherwise competent technophiles curl into a prenatal position while rocking back and forth. It is impossible for a reader to study this entire book and not be able to participate in an intelligent conversation on radar systems from both a technical standpoint and a historical perspective. Can you tell I'm impressed?
Note: Some of these books are available as prizes in the monthly RF Cafe Giveaway; however, this one will likely remain in my possession for a long time.
1. What is another, more common name for a coherent radar system?
c) Pulse Doppler radar
Both amplitude and phase are measured, thus enabling calculation of speed.
(see page 15)
2. Match the following common radar frequency bands with their central frequencies.
a) C - 3 GHz
b) Ka - 38 GHz
c) Ku - 15 GHz
d) S - 6 GHz
e) X - 10 GHz
(see page 99)
3. In which direction(s) does(do) an antenna's sidelobe(s) exist?
d) All directions
Sidelobes are due to non-ideal physical limitations of an antenna and its environment (ground integrity, antenna support, nearby structures, etc.).
(see page 115)
4. What causes a grating lobe in an electronically scanned antenna?
a) Spacing radiating elements at too great of a distance
Spacing the radiating elements is determined by the equation d < (1 + sin θ), which yields the distance of the grating lobe from the main lobe.
(see page 135)
5. What is the main advantage of a pulsed radar over a non-pulsed radar?
a) Pulsed radar avoids transmitter leakage issues
Pulsed radars emit a signal and then turn off the transmitter while listening for returns. Doing so eliminates having to contend with strong inband signals that will raise the noise floor and lower the minimum detectable signal.
(see page 149)
6. What is radar cross-section (RCS)?
d) Effective area of the signal reflecting surface relative to a perfectly reflecting sphere of the same projected area
Radar cross-section is a form of quality parameter that represents how large of an effective target a body appears relative to a perfectly reflective sphere. Something like an unpainted Cessna Citation bizjet will have a large RCS since there are so many large, reflective surfaces whereas an F-117 Stealth Fighter with its multifaceted surfaces with non-reflective paint will have a small RCS. Keep in mind, BTW, that the Cessna wants to be painted strongly by radar for air safety reasons.
(see page 166)
7. What is "chirping?"
b) Linear frequency modulation over the transmit time
Chirping gets its name from the sound hear when a linearly swept audio frequencies is broadcast through a speaker - it sounds like a bird's chirp.
(see page 233)
8. Coherence in radar refers to which of the following?
d) Phase of the signal from one pulse to the next
Phase coherence is relative to a reference oscillator that is used to measure time between the transmitted pulse and the return signal.
(see page 269)
9. What defines bins in a Doppler filter bank?
a) Frequency bands
Return signal information (phase, amplitude, angle of arrival, etc.) is sorted according to a prescribed range of frequencies that indicate what the Doppler shift - positive or negative - is relative to the transmitted signal.
(see page 297)
10. What is the name for unwanted returns from the ground?
Clutter is an undesirable return signal that obscures the desired target. Moving target indication (MTI) can cancel stationary targets to reveal moving targets that would otherwise be undetectable.
(see page 329)
11. Match each picture with what it represents.
A - High accuracy, low precision
B - Low accuracy, high precision
C - High accuracy, high precision
D - Low accuracy, low precision
Accuracy is keeping all the hits centered around the intended target. Precision is clustering all of the hits in the same area even if not centered on the target.
(see page 433)
12. Which type of pulse repetition frequency (PRF) results in unambiguous range?
A low PRF assures that the echo from the farthest distance target arrives prior to the transmitting of the next pulse, thus eliminating 'second-time-around' responses.
(see page 383)